Abstract: Cracks have occurred in the welding joint of the flange, which is matched with a blowdown relief valve of a gas collecting device. The reason of crack was analyzed by means of macro and micro examination, chemical composition analysis, mechnical property testing and so on. The results showed that the sulfide stress corrosion cracking had occurred, and improper welding procedure or excessive cooling rate after welding was the main reason leading to the susceptive microstructure of sulfide stress corrosion cracking which resulted in the cracks in the weld.